High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al., U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) or PET has become an important constituent of injection moldable compositions. Further, poly(1,4-butylene terephthalate) or PBT, because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Work pieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss and lower surface friction.
Furthermore, in particular, poly(1,4-butylene terephthalate) is much simpler to use in injection molding techniques than poly(ethylene terephthalate). For example, it is possible to injection mold poly(1,4-butylene terephthalate) at low mold temperatures of from about 30.degree. to 60.degree. C. to produce highly crystalline, dimensionally stable moldings in short cycle times. Because of the high rate of crystallization, even at low temperatures, no difficulty is encountered in removing the moldings from the molds. Additionally, the dimensional stability of poly(1,4-butylene terephthalate) injection moldings is very good even at temperatures near or well above the glass temperature of poly(1,4-butylene terephthalate).
However, the impact resistance of unmodified polyesters is relatively low at room temperature and below. Thus for many applications, it is desirable to have polyesters which are impact resistant at relatively high and relatively low ambient temperatures. Yet, the other mechanical properties such as modulus of elasticity, tensile strength at yield and at break should be impaired either not at all or only to an acceptable degree.
It has been recommended in various places to improve the impact resistance of polyesters by adding other polymers including interpolymers and copolymers. Specifically, the impact strength of thermoplastic linear crystalline polyesters, including poly(1,4-butylene terephthalate), has been improved by the incorporation therein of an ethylene-propylene nonconjugated diene rubbery terpolymer (EPDM). Although EPDM is capable of impact-modifying BPT polyester compositions, e.g., Coran et al., U.S. Pat. No. 4,141,863 and Tanaka et al., U.S. Pat. No. 4,290,927, such compositions often suffer from "incompatibility" resulting in streaks or delamination of molded or extruded parts.
In Hepp, European Pat. application No. 0 149 192, published July 24, 1985, there is disclosed a thermoplastic molding composition consisting of a thermoplastic resin, e.g., polyester, copolyester or block copolyester and an EPDM epoxidized with, e.g., m-chloroperozy-benzoic acid. The examples given by this reference in Tables 1, 2 and 3 do not, however, exhibit a combination of good impact strength and acceptable knit-line characteristics.
Siegel, U.S. Pat. No. 3,769,260 discloses that a functionalized rubber improves impact strength of polyesters, and suggests a range of 0.02 to 20 microns in diameter for the dispersed rubber phase particles.
Epstein, U.S. Pat. No. 4,172,859, discloses the use of random copolymers containing various polar monomers. He also alludes to the use of materials grafted with various polar monomers, e.g., glycidyl methacrylate (GMA), to impact modify thermoplastic polyesters including PBT and PET. However, this patent does not deal with and therefore fails to recognize several factors that are critical to the function of EPDM-g-GMA materials as impact modifiers for PBT systems. First, it does not recognize the benefits of reactive glycidyl (meth)acrylates as graft monomers over non-reactive polar monomers such as maleic anhydride or n-vinyl pyrrolidone. Second, in Col. 5, lines 35-36, Epstein discounts crosslinking (gel formation) as unnecessary to the function of such modifiers, and states that crosslinking may be harmful. Third, the Epstein patent does not teach the need for a specific rang of grafted GMA content in the rubber. Finally, Epstein discloses modifier particle sizes between 0.01-3.0 micron, and expressly states a distinct criticality for particle size substantially entirely below 1 micron in size.
In Olivier, U.S. Pat. application, Ser. No. 690,613 abandoned, filed Jan. 11, 1985, there are disclosed glycidyl methacrylate grafted EPDM impact modifiers for polyester resins. In the specific examples, he describes materials having gel contents of less than 5 percent or more than 48.7, with nothing in between. With little or no gel contents, i.e., 5 percent or less, he states that thermoplastic polyester compositions with high impact strengths are obtained. With high gel contents, specifically above 48.7%, Olivier discloses that the material was a much poorer impact modifier than one which was free of gel. In any event, the specific examples have subsequently been found not to provide an acceptable combination of good impact strength and knit-line characteristics. Moreover, no mention is made of any effect of impact modifier particle size on these important characteristics.
It has now been unexpectedly discovered that thermoplastic polyester compositions comprising glycidyl methacrylate grafted EPDM (EPDM-g-GMA) impact modifiers can be prepared that possess high impact strength combined with good knit-line characteristics if careful attention is paid to graft monomer content, gel content, and rubber particle size in the impact modifier. The gel content preferably used is especially unexpected because it happens to fall in a range not actually investigated by Olivier. Compositions with these important properties have been developed as a result of numerous trials which have established that it is required to use EPDM-g-GMA materials with greater than 2.0 percent, preferably greater than 3 percent, and especially preferably greater than 4 percent, GMA contents, and gel contents in the 10-80 percent range. Impact properties are also vastly improved in thermoplastic blends with well-dispersed rubber particles in which more than 60 percent, preferably more than 70 percent by weight of the rubber particles are 1 micron or larger in diameter. Nothing in the prior art renders such parameters obvious, and they cannot be developed by mere routine optimization experiments.